An essential semiconductor device is semiconductor memory, such as a random access memory (RAM) device. A RAM allows a memory circuit to execute both read and write operations on its memory cells. Typical examples of RAM devices include dynamic random access memory (DRAM) and static random access memory (SRAM).
Another form of memory is the content addressable memory (CAM) device. A conventional CAM is viewed as a static storage device constructed of modified RAM cells. A CAM is a memory device that accelerates any application requiring fast searches of a database, list, or pattern, such as in database machines, image or voice recognition, or computer and communication networks. CAMs provide benefits over other memory search algorithms by simultaneously comparing the desired information (i.e., data in the comparand register) against the entire list of pre-stored entries. As a result of their unique searching algorithm, CAM devices are frequently employed in network equipment, particularly routers, gateways and switches, computer systems and other devices that require rapid content searching, such as routing tables for data networks or matching URLs. Some of these tables are “learned” from the data passing through the network. Other tables, however, are fixed tables that are loaded into the CAM by a system controller. These fixed tables reside in the CAM for a relatively long period of time. A word in a CAM is typically very large and can be 96 bits or more.
Practically all digital networks make use of some form of packet or block type data format to dynamically route data packets or blocks through the network. The data contained in the packets can be categorized in various ways, including type of packet, packet content, size, creation date, and urgency of delivery, for example. Depending on the purpose of the communications system and the preferences of the user, it may be necessary to limit or expand the amount of bandwidth to be allocated to a particular memory of dRAMs can be particularly beneficial in applications involving resource allocation. For example, when system capacity limitations restrict the amount of data that can be transmitted by the network, or if a user wishes to give priority to certain categories of data over others, CAMS may be used to prioritize the flow of data.
CAMs are also used in communications systems as search engines for routing paths in data network routers and switches. The packets being routed can be viewed as belonging to a particular category of data which, in turn, can impact on how high (or low) a priority the data is assigned and how much bandwidth a user wants to devote to the data. Typically, a CAM issues a single search result that is independent of a category to which the packet belongs. Consequently, it is necessary for the user to handle bandwidth allocation, for example, by assigning categories to CAM searches and transmitting each category to an assigned priority encoder for further processing. A more efficient way of utilizing CAMs as a search engine is needed.